In U.S. Pat. No. 5,291,501, Hanna describes a double-clad fiber laser in which a single mode central core is doped with a first laser active material, such as thullium (Tm.sup.3+), a multimode inner cladding surrounding the core is doped with a second laser active material, such as neodymium (Nd.sup.3+), and an outer cladding surrounds the inner cladding. The three fiber regions, i.e. the central core, inner cladding and outer cladding, are arranged concentrically. The inner cladding is optically pumped with a multimode laser diode array pump source, while the central core is pumped by the laser emission from the inner cladding.
In U.S. Pat. Nos. 5,087,108 and 5,218,665, Grasso et al. describe an erbium-doped fiber amplifier employing an active core doped with erbium and receiving both pump (.lambda..sub.p =980 nm) and signal (.lambda..sub.s =1550 nm) light, and a passive core absorbing spontaneous light emissions (.lambda..apprxeq.1536 nm) from the active core. The two cores are arranged parallel to each other and are constructed so that the spontaneously emitted radiation leaks out of the active core into the passive core. In this way, the spontaneous emission which corresponds to a narrow emission peak of great intensity in the gain spectrum of erbium can be attenuated, so that the signal light which is amplified can be at a wavelength corresponding to a lower intensity but broader portion of the gain spectrum. The wavelength tolerances for the semiconductor lasers selected to provide the signal light need not, therefore, be as stringent.
One problem limiting the output power and efficiency of fiber lasers and amplifiers is stimulated Brillouin scattering. This nonlinear optical effect results from the interaction of the laser light with acoustic waves in the fiber medium through which the laser light is propagating, producing inelastic backscattering of the light together with a frequency shift equal to the frequency of the acoustic waves. The backward propagating light is amplified at the expense of the forward propagating light. Further, the acoustic waves may also be amplified by this effect, generating an acoustic intensity that can easily damage the fiber medium. These deleterious effects limit the useful light output intensities that can be produced in a laser or amplifier fiber core.
It is an object of the invention to provide an optical fiber laser or amplifier medium with increased output power without producing nonlinear optical effects such as Brillouin scattering.